[stokes] restore reference solution

The changes introduced in commit 14916 had no effect on the test
results. However, the reference solution was also updated. This restores
the old reference solution.

Discussed with fetzer.
 


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15074 2fb0f335-1f38-0410-981e-8018bf24f1b0